Gearboxes

[Plc_User]

I always thought that gearboxes with big gear ratios (especially ratios of > 80) and that have an angle of 90 degrees between in- and out shaft are selfbraking (you can't drive the motor by pulling the outgoing shaft).
But recently in an installation we had a 90 degrees gearbox of gearratio 60 that was completely selfbraking, and another gearbox of 90 degrees and gearratio of 180 where I could rotate the outgoing shaft by hand.
What makes a gearbox selfbraking? In the past we more then once had a problem where we wanted a roller to be drive by a certain torque but the precision was inaccurate because of the ratio of the gearbox?

 

[milldrone]

I always thought that gearboxes with big gear ratios (especially ratios of > 80) and that have an angle of 90 degrees between in- and out shaft are selfbraking (you can't drive the motor by pulling the outgoing shaft).

Gee. I always thought the magic number was greater than 15 to 1 in a single reduction and the worm (has to be a worm not a hypoid or bevel) needed to be a single start configuration. Perhaps I have been listening to the wrong urban legends!

What makes a gearbox selfbraking?

The internal friction is greater than the reverse torque at the output shaft.

In the past we more then once had a problem where we wanted a roller to be drive by a certain torque but the precision was inaccurate because of the ratio of the gearbox?

If you are speaking of worm drive boxes, they have large amounts of internal friction.

 

[Tom Jenkins]

There are multiple types of right angle gear boxes. Worm gears are the most common, but there are also bevel gear types.

Generally a worm gear with a ratio of 30:1 or more will be "self locking". This means that the friction between the worm and the gear is high enough that you can't back drive it - that is, you can't rotate the output shaft to make the input shaft rotate.

The degree of self locking and the ratio where it occurs is a function of gear material, gear quality, and lubrication.

Never ever ever count on the self locking feature to hold a load against gravity or an external force, particularly if safety is involved. An example of a big no-no would be using the worm gear reducer's resistance to back driving to hold up an elevator. Vibration may cause loss of friction and allow the load to slip. A worm gear is NOT intended to be a brake.

 

[Plc_User]

Do manufacturers have a property that indicates the rate of friction loss on the gearbox? Can you see from the documentation how much the loss is? Do you have concrete examples on product sheets? Thanks

 

[leitmotif]

Do manufacturers have a property that indicates the rate of friction loss on the gearbox? Can you see from the documentation how much the loss is? Do you have concrete examples on product sheets? Thanks

I have never seen friction mentioned on gearboxes. Have seen percent efficiency or cooling requirements listed. Maybe you can work back from this kind of nember to get friction - or just measure it with a torque wrench on input shaft.

Worm gears are reputed to never be able to drive backwards - thus are used for holding a load. Works well when new but when gears wear or load is too high then load can drive unit. One of two things will happen - either tear up gear box or the load comes down slowly (which was a blessing in one case I have observed). Loads should ONLY be held in place with a brake or if very heavy or safety issue a pawl mechanism.

Dan Bentler